If I have to file a charge, can I ask a mediator to testify regarding the bargaining sessions, or during a grievance mediation?

Under no circumstances will a mediator testify at any proceeding (court, administrative, or arbitration) about any discussions that occurred during mediation. This is the case with collective bargaining mediation, or mediation of grievances, or employment disputes. Even if a subpoena is served, the FMCS will move to quash the subpoena and will engage in every effort to avoid testimony of mediators at any proceeding.